Force each new render into sequential render slot
here is a script that switch the render slot automatically after each render 
-This is for blender 2.72 and earlier versions :

import bpy
from bpy.app.handlers import persistent

@persistent
def PostRender(self):
        bpy.data.images['Render Result'].render_slot += 1
        bpy.data.images['Render Result'].render_slot %= 7

bpy.app.handlers.render_complete.append(PostRender)


paste and run inside blender



as Ray Mairlot noted the changes in blender 2.73 and later versions require replacing render_slot with render_slots.active_index as follows :

import bpy
from bpy.app.handlers import persistent

@persistent
def PostRender(self):
        bpy.data.images['Render Result'].render_slots.active_index += 1
        bpy.data.images['Render Result'].render_slots.active_index %= 7

bpy.app.handlers.render_complete.append(PostRender)